Williams signed on with the Panasonic Wild Knights last month but New Zealand asked the club to release its high-profile recruit so he could cover for the injured Conrad Smith in two Championship Tests against the Wallabies.
With less than a week until the opening match, there were doubts Williams would be available as the Japanese club insisted on medical tests to ensure he was fully fit after a taxing season with Super 15 winners the Chiefs.
But the New Zealand Rugby Union (NZRU) said on Monday that all arrangements had been made for Williams, 26, to again don the black jersey and it could confirm his place in the team.
"It's what we expected, but we knew it was going to take a bit of time, and we are now all looking forward to preparing for Saturday's challenge," All Blacks coach Steve Hansen said in a statement.
The decision means Williams will play Australia in Sydney next Saturday and Auckland a week later.
After completing a lucrative stint in Japan, Williams is expected to return to Australia's National Rugby League next year, the competition where he made his name with a 2004 premiership win with the Canterbury Bulldogs in his rookie season.